I agree with much of what you say about word patterns,knowing the story of the song,tune etc, I use much the same tools myself but occasionally where the next line should be is a hole,sometimes you can talk your way out of the problem or bluff in some way,other times not so. I occasionally use a crib sheet as a prompt or safety net if I decide to do something, I rarely do at short notice and do not have the time to rehearse it and refresh my memory. As I said before I have little problem with the discreet use of such a thing,only when the device intrudes noticeably into the performance. I liken them to a bad
accompaniment. As you so rightly say any performance should be at least competent.